Two popular techniques currently in use for optical ranging of a target surface are known, respectively, as the standard optical triangulation system and the Biris (bi-iris) system, the latter employing an apertured mask in a converging lens system of an imaging device having a position sensitive detector, e.g. a CCD camera.
These systems are described and compared in F. Blais et al. (88), i.e. "Practical Considerations for a Design of a High Precision 3-D Laser Scanner System", published in Optomechanical and Electro-optical Design of Industrial Systems, SPIE Vol. 959, 1988, pp 225-246, and also in F. Blais et al. (91), i.e. "Optical Range Image Acquisition for the Navigation of a Mobile Robot", published in the Proceedings of the 1991 IEEE International Conference on Robotics and Automation, Sacramento, California, Apr. 9-11, 1991. The Biris system had previously been reported by M. Rioux et al. (86) in "Compact Three-Dimensional Camera For Robotic Applications", published in the Journal of the Optical Society of America A, Vol. 3, p 1518, September 1986, and in M. Rioux U.S. Pat. No. 4,645,347 issued Feb. 24, 1987. All these documents are hereby incorporated herein by reference.
An advantage of the Biris system is its small size and the robustness of the range sensor. On the other hand, the triangulation system can achieve greater accuracy than the Biris system, but is very sensitive to the condition of the environment. It operates well in a highly controlled environment, but fails in more complex situations. For example, if the target surface is reflective and reflects light onto one or more non-target objects in the vicinity of the target surface, or there are other perturbations, such as reflections or light from an open window, that enter the field of view of the camera, the detector becomes confused and cannot discriminate between the true target surface and false signals from the other objects or perturbations. The Biris system is better than the triangulation system in this respect, because it uses the redundancy introduced by an apertured mask to validate the measurements, but nevertheless in a severely cluttered environment both systems are subject to error. FIG. 2 of Blais et al. (91) shows the difference between the images obtained by the respective systems in a cluttered environment.